Yeah, it could take a few hours (or even days) for all mirrors to sync across the world. Changing the server in Software & Updates to point to Main server and refresh your sources might accelerate things.
At this time, it's okay to use the -d flag since 20.04 was just released and this just bypasses the check to stop users upgrading to what would normally be a development branch.
If I remember, users on 18.04 won't see an invitation to upgrade to 20.04 until the first poin…

So actually, you're right in that we need -d. Though, the -d won't be necessary once [somebody or robot] at Canonical HQ enables updates to show in Software Updater. Currently it doesn't show for me in 19.10, and I know LTS releases wait for the first point release (.1)

As things stand right now - I'm extremely happy with 20.04. Definitely will be leaving this on my main box for a while (although, I'll be honest and probably upgrade to 20.10 in November!)
Big Thumbs up to everyone involved with Ubuntu-Mate for a great edition!